Unbacked SymInt fixes for subclasses + data-dependent slice() bounds (non-dynamic) #143526
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This was referenced Dec 18, 2024
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…142062) Related: #125914 (specifically see [comment](#125914 (comment))) This PR addresses two broken things involving the usage of unbacked SymInts for calls to `slice()` with data-dependent bounds. These issues are encountered in practice for `narrow()` operating on the batch dim with an NJT input, but apply to other subclasses as well. The test in this PR uses a purpose-built subclass. There are two different issues here, depending on whether `torch.compile()` is called with `dynamic=True`. In practice, these only occur when the unbacked SymInts are created within the torch_dispatch implementation of a subclass, because the unbacked symbols are considered "freshly created" when the output subclass instance is handled in Dynamo. **Error 1 (dynamic=False):** ``` LoweringException: GuardOnDataDependentSymNode: Could not guard on data-dependent expression Eq(-Min(22, Max(0, u0)) + Min(22, Max(u0 + u1, Max(0, u0))), 0) (unhinted: Eq(-Min(s0, Max(0, u0)) + Min(s0, Max(u0 + u1, Max(0, u0))), 0)). (Size-like symbols: u1, u0) ``` The expression comes from the use of `clamp()` logic for `SliceView` in Inductor: https://github.com/pytorch/pytorch/blob/41e59754b407533b060b874c22ca4feda38bd83a/torch/_inductor/ir.py#L3014 If the (start, end) bounds for the `slice()` are statically known to be in range for the given dim (e.g. provided via `torch._check()` calls), we can avoid this `clamp()` logic and the error. This PR implements this fix. **Error 2 (dynamic=True):** ``` torch._dynamo.exc.InternalTorchDynamoError: PendingUnbackedSymbolNotFound: Pending unbacked symbols {u0} not in returned outputs NestedTensor(size=(2, s16, s1), offsets=FakeTensor(..., device='cuda:0', size=(3,), dtype=torch.int64), grad_fn=<NarrowBackwardAutogradNestedTensor0 object at 0x7f1f8603cfd0>, contiguous=True) ((s1*s16, s1, 1), s1*u0) ``` The storage offset of the values component of the returned NJT is `s1*u0` where `s1` is known to be an integer. This PR expands the special logic handling the `constant * u0` case to handle SymInts as well: https://github.com/pytorch/pytorch/blob/314e08eb52ad0e9b1c3eb6e149ec8a452e05b9c3/torch/fx/experimental/symbolic_shapes.py#L1013-L1031 Pull Request resolved: #142062 Approved by: https://github.com/ezyang ghstack dependencies: #143526
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Stack from ghstack (oldest at bottom):
Lifted non-controversial (non-dynamic) fixes from #142062. See description there for context.
cc @ezyang @SherlockNoMad @EikanWang @jgong5 @wenzhe-nrv @voznesenskym @penguinwu @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @jiayisunx @ipiszy @yf225 @chenyang78 @kadeng @muchulee8 @ColinPeppler @amjames @desertfire @chauhang @aakhundov